In any mobile communication system, such as a Global System for Mobile communications (GSM) network, active calls conducted between a mobile station (MS) and a base station need to be handed over to a different base station as the mobile station moves between different coverage areas, or cells. Depending on how each cell is defined, handover may require the active call to be re-routed simply through a different base station transceiver (BTS), through a different base station controller (BSC) or through a different mobile services switching center (MSC). Handover may also be necessary when capacity problems are met in any one cell.
Handover necessitates a certain amount of operation and maintenance activities on installation of a system, such as defining neighboring cells, as well as the BSC and MSC that controls the cell, defining which cell frequencies should be measured and what threshold value to use to initiate handover. In a conventional GSM network the BSC sends a MS a list of predetermined frequencies to be measured. Two lists may be sent out, a first list being used for idle mode, such as when the MS is roaming, and a second used for active mode when a call is ongoing. This second list defines which frequencies the MS should measure and report back on. These lists contain a set of values that refer to absolute radio frequency channel numbers (ARFCN) of neighboring cells. In addition to these frequency channel numbers, the BSC also knows base station identity codes (BSIC) of all neighbouring cells. The MS measures the frequencies defined by these channel numbers and reports these measurements to the BSC. In practice, the MS will report on only the six best measurement values and only for those cell frequencies with which the MS can synchronize and consequently receive a BSIC. The measurement report sent back to the BSC by the MS includes a reference to the ARFCN, the BSIC and an indication of the received downlink signal strength. In fact the report does not specify the exact ARFCN but rather refers to the position this number occupied in the measurement list. On the basis of this report, the BSC decides whether handover is necessary and to which cell. The initiation of handover is performed according to the standard GSM mechanism for each vendor. Specifically, a message is sent by the base station controller to the MSC connected to the BSC indicating that handover is required. This message contains a cell identifier, encompassed in a cell global identity (CGI), which defines the mobile country code, mobile network code, location area code and cell identifier for the cell to which handover is requested. The CGI is fetched by the BSC from a list using the BSIC and ARFCN obtained for the cell. With this CGI the MSC is able to determine which other MSC handles the cell defined by the CGI value.
Recently proposals have been made to extend conventional cellular networks by including access networks that utilize a low power unlicensed-radio interface to communicate with MSs. The unlicensed mobile access (UMA) networks (UMANs) are designed to be used together with the core elements of a standard public mobile network and consist essentially of plug-in low-power unlicensed radio transceivers, or access points (AP), each AP designed to establish an unlicensed radio link with a MS and a controller or interface node connecting the unlicensed radio transceivers with the mobile core network. Suitable unlicensed-radio formats include digital enhanced cordless telecommunications (DECT), wireless local area network (WLAN) and Bluetooth. An adapted mobile handset capable of operating over both the standard air interface (e.g., the Um interface) and the unlicensed-radio interface means that the subscriber requires only one phone for all environments. The UMA network is constructed so that the core elements, such as the MSCs, of the public mobile network views the interface node as a conventional BSC. Such a UMA network and the MS for use with this UMA network are described in various UMA standard specifications and other implementations, such as European patent application No. EP-A-1 207 708. The content of this application is incorporated herein by reference.
The low power and resultant low range of the unlicensed-radio interface means that several such UMA networks may be provided in relatively close proximity, for example one access network per floor of an office building or in a private home. The connection between the unlicensed-radio transceivers and the associated unlicensed network controller (UNC) is provided by a fixed broadband network. Preferably, communication over this network uses the internet protocol (IP), which greatly facilitates the installation of the UMA network, permitting a subscriber to plug-in an unlicensed-radio transceiver or in his own home and consequently install an unlicensed-radio access point (AP) himself. However, the flexibility of such UMA networks also presents difficulties. Since an access point can be freely installed and moved by a subscriber to a separate city, state or even country, yet still connect to its original UNC, the exact location of the AP cannot always be tracked by the core network. Massive cell planning by the operator is required for the core network to track the location of the AP. This imposes huge demands on the operation and maintenance activities required for handover to and from the UMA network, as neighboring cells may change frequently. Also billing restraints in some areas may require the re-assignment of a relocated AP to a more appropriate UNC, particularly if revenue from calls originating from a specific AP must be accounted for in a specific region of a country. In view of the small size of the AP to an UMA network, it would not be cost effective for operators to configure each AP separately. However, it is also undesirable to leave the task of configuration to the subscriber as such a solution would be error prone and consequently unreliable. In addition, suppliers would not wish to configure AP differently depending on where these are ultimately to be installed.
Moreover, current UMA specifications presume that a single ARFCN value and a single BSIC value are defined for the UNC. This creates some limitations in the current procedures for the handover of a MS into UMAN that arise in some scenarios where ARFCN's are a scarce resource. For example, such scenarios occur when one UNC is serving multiple MSC Serving Areas and the same ARFCN value cannot be used in the BSC's and cells belonging to these MSC Service Areas. As illustrated in the logical diagram of FIG. 4, MSC-1 400 and MSC-2 404 are both connected to UNC 402. MSC-1 400 only has one free ARFCN value X available in the Cells belonging to MSC-1 400. Similarly, MSC-2 404 only has one free ARFCN value Y available in the Cells belonging to MSC-2 404. As a result, UNC 402 is not able to indicate different ARFCN values to MSs registering from Service Areas covered by MSC-1 400 and MSC-2 404. Accordingly, handover of the MS to the UNC 402 is not possible in this scenario.